Why am I crying?

A vagrant is about to be hit by a train. You can save him, but you have perfect information that you will be giving up your own life for his. You know that if you were to live, you would be more productive to society than he would. There is no uncertainty about this; your choice hinges solely on your value system and how those values dictate your actions. Would you do it?

This is an unfair question to ask; the person being asked to answer it knows that his answer will directly imply something about his character and his honesty. If he answers without hesitation, he feels dishonest; if he answers that he wouldn't, he feels guilty of being heartless, even though his answer makes economic sense.

I knew the gravity of this question when I posed it to somebody today. Neither of us were able to give a straight answer.